This invention relates to X-ray detectors, and more particularly to solid state detectors having improved radiation characteristics including decreased decay time.
Solid state scintillator materials are in common use as components of radiation detectors in X-ray detection apparatus such as counters, image intensifiers and computerized tomography (CT) scanners. One embodiment of the present generation of solid state ceramic scintillators comprises oxide mixtures in which a rare earth oxide is present as an activator, along with various combined matrix elements which are also usually rare earth oxides. Other combined metals may also be present as additives for specific purposes. These scintillators have been characterized by the advantageous properties of high efficiency, moderate decay time, low afterglow and little or no radiation damage upon exposure to high X-ray doses.
Scan times of CT systems are related to primary decay time (sometimes simply "decay time" hereinafter) of the scintillator roughly by a factor of 1,000. Thus, a scintillator having a decay time of 1 millisecond will typically produce a scan time of about 1 second. The scanning units containing the present generation of scintillators have scan times on the order of 1 second, and in any event no lower than about 0.7 second.
In future generations of CT scanners and the like, shorter scan times are desired. This is true because decreasing scan time makes possible an increase in patient volume covered in a given time, usually a single breath hold. Also, it reduces image blurring due to motion of internal organs and of non-cooperating patients, including pediatric patients.
Scan times of this magnitude may be achievable if the primary decay time of the scanner is shortened. In general, scan time in seconds is associated with a primary decay time of an equal number of milliseconds. A decay time on the order of 0.25 millisecond, corresponding to a scan time of 0.25 second, would maximize the ability of the scanner to "keep up" with the data supplied to it. However, any measurable percentage decrease in decay time from that exhibited by the present generation of ceramic scintillators would be a distinct improvement, particularly when accompanied by the other advantageous properties described above.
Among the preferred scintillator compositions in the present generation of CT scanners are the ceramic scintillators employing at least one of the oxides of lutetium, yttrium and gadolinium as matrix materials. These are described in detail, for example, in U.S. Pat. Nos. 4,421,671, 4,473,513, 4,525,628 and 4,783,596. They typically comprise a major proportion of yttria (i.e., Y.sub.2 O.sub.3), up to about 50 mole percent gadolinia (Gd.sub.2 O.sub.3) and a minor activating proportion (typically about 0.02-12, preferably about 1-6 and most preferably about 3 mole percent) of a rare earth activator oxide. Suitable activator oxides, as described in the aforementioned patents, include the oxides of europium, neodymium, ytterbium, dysprosium, terbium and praseodymium. Europium-activated scintillators are often preferred in commercial X-ray detectors by reason of their high luminescent efficiency, low afterglow level and other favorable characteristics. Europium is typically present therein in amounts up to 30 and most often up to about 12, preferably in the range of 1-6 and most preferably about 3 mole percent. Decay times of such scintillators are on the order of 0.9-1.0 millisecond.
The search thus continues for ceramic scintillator compositions having shorter decay times in combination with the aforementioned other advantageous properties.